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My 10 hp RPC just crapped out so am considering a VFD to replace it. I was using the RPC to power my 16" South Bend and my J head Bridgeport mill (never at the same time). The lathe has a 3 hp motor and the mill has a 1-1/2 hp motor. Am considering a 3 hp VFD to be used with both machines. Don't plan on using the VFD to regulate speed just to provide 3 phase power. Can the VFD be wired so that it will work with either machine or would I need a separate VFD for each machine? Also would prefer American made VFD so would appreciate recommendations.

While it would be possible to wire a single VFD to run either motor, that would certainly not be my first choice, especially given the difference in HP. I would recommend a properly sized one for each machine. Once you have the variable speed available you will wonder how you got along without it, even with a variable speed head on the mill if equipped.

H-M Supporter - Gold Member

I have a G2 and both my machines are tied into it. Works just fine. I put a safety interlock (two 3 pole contactors) and a selector switch only allowing one to run at a time. You could do it with plugs fairly easy as well. I’m not familiar with any US made frequency drives I suppose Allen Bradley maybe but I’d assume all the components are from China or Japan.

H-M Supporter - Gold Member

As long as it is not one of the VFD's that needs to know all about the motor it drives, I do not see an issue. I have swapped VFD's from one motor to another, without any reprogramming. Right know, I temporarily borrowed one from a drill press, to use on my new to me Sothbend 16".

Its probably just paranoid, and a waste of money, but I have all my VFD's rated for twice the motors rating (3HP VFD, powering a 1.5HP motor). For the little difference in price, I am thinking it will help if the VFD is not working near its capacity.

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I agree with Jim in this case for several reasons, but with one caveat. Using the same VFD for both machines using the same parameters is not a good choice. What can be done is some VFDs allow you to setup two separate parameter files, one for each motor, you can switch the motor parameter files based on a programmed input. You cannot use the machine motor switch/contactors to control the VFD output, you need to program the VFD low voltage inputs for the VFD run commands. Since the run commands and mode of operation would be different between the lathe and the mill, it can get messy quickly. At the end of the day if this is a long term replacement you would be better off using a dedicated VFD for each machine. Something like the 2 Hp Teco L510 ($175) on the mill, or better yet a GS3, WJ200 or other mid priced VFD would be better suited for the lathe. The Teco L510 cannot accommodate an external braking resistor which is a requirement on a lathe in particular of that size.

If you are looking for a drop in fixed 60 Hz single phase to 3 phase system, there is the Phase Perfect converters, but they are very expensive, so more practical for powering multiple machine or CNC machines.

I am not sure if any VFDs are American made, and most of the components will be offshore, although you can go with a US brand like ABB which have a good reputation. I mostly install/work with the Hitachi WJ200 and the Yaskawa V1000 series. Both are widely used, along with the Automation Direct (GS3), and probably have better support for these brands from others in this forum. Automation Direct is a great US company and very good technical support. At the end of the day, if you want just a plug and play solution and do not need the VFD features, it is probably both more practical and cost effective to get/build a 5 or 7.5 Hp replacement RPC.

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For markba633csi I took the pony motor to the repair shop today to have it tested. If it needs to be rewound it would cost >$400. Won't be putting that amount of money in it as I can find a used one for well less than that. Still haven't decided if I will stay with a RPC or will go with a VFD (or 2). The RPC has worked well for me for the last several years and I can easily switch from lathe to mill with no problem. Will have to make a decision in the next few days.

A special thanks to all who have provided input to my question, you guys are great.

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Got a call from the motor shop informing me that there is no problem with the pony motor of my RPC. Guess the problem lies elsewhere. Will hook everything back up tomorrow and thoroughly check all other components and wiring. Wish me luck. Again thanks for all the informed information.

H-M Supporter - Gold Member

I agree with Jim in this case for several reasons, but with one caveat. Using the same VFD for both machines using the same parameters is not a good choice. What can be done is some VFDs allow you to setup two separate parameter files, one for each motor, you can switch the motor parameter files based on a programmed input. You cannot use the machine motor switch/contactors to control the VFD output, you need to program the VFD low voltage inputs for the VFD run commands. Since the run commands and mode of operation would be different between the lathe and the mill, it can get messy quickly. At the end of the day if this is a long term replacement you would be better off using a dedicated VFD for each machine.

I have a couple questions. I set my perameter for what I had found best lathe 8 second ramp and a 3 second deceleration. Will this adversely effect my mill? I typically only adjust the frequency when threading to a shoulder would there be any issues with that? Also on my gs2 I have just use the two wire forward and reverse using a 24v DI for the forward command and the same for the reverse. I’m breaking the common for my E-stop. Most my day to day drive experiences are 1500HP ABB and Siemens VFDs. My big motors are 2000HP traction motors driven with SCRs. I’ve not had any issues with my current set up just kinda worried now. I never run my mill at less than 60hz the motor on it is not drive rated. I just don’t want the ramp and deceleration to crater my mill.

It might be a bit slow for the mill, but won't hurt anything. I think I have mine set at 3 and 3. You just have to think ahead a bit when power tapping on the mill.

I typically only adjust the frequency when threading to a shoulder would there be any issues with that? Also on my gs2 I have just use the two wire forward and reverse using a 24v DI for the forward command and the same for the reverse. I’m breaking the common for my E-stop.

H-M Supporter - Silver Member

I have a couple questions. I set my perameter for what I had found best lathe 8 second ramp and a 3 second deceleration. Will this adversely effect my mill? I typically only adjust the frequency when threading to a shoulder would there be any issues with that? Also on my gs2 I have just use the two wire forward and reverse using a 24v DI for the forward command and the same for the reverse. I’m breaking the common for my E-stop. Most my day to day drive experiences are 1500HP ABB and Siemens VFDs. My big motors are 2000HP traction motors driven with SCRs. I’ve not had any issues with my current set up just kinda worried now. I never run my mill at less than 60hz the motor on it is not drive rated. I just don’t want the ramp and deceleration to crater my mill.

Your ramp times should not be an issue. The lower frequencies generate heat with less motor motion to cool, but for such short times, it really doesn't matter. You also are not pulling load while turning the spindle on, so even better. The ramp down time adds heat back into the VFD. If you try to stop too fast, you will fault the VFD. To remedy this you can add a braking resistor. I would think you should be able to be a lot more aggressive with your ramp times.

H-M Supporter - Silver Member

Typically for *best* performance, a VFD needs an intimate knowledge of the motor and load it is driving. If you have 2 identical motors, switching between them shouldn't be an issue. The issue comes when the drive is shared between dissimilar motors as the cornering frequency, slip speed, and motor inductance can vary greatly. What this can result in is higher heat generation, poor speed control, and incorrect application of control strategies. Some drives allow you to save configurations for multiple motors (you may be familiar with this from the AB PowerFlex drives) and load them as needed. Another option is to wire the motors in parallel and tune the drive for the combined motor load, but this probably won't work well as your lathe would turn on when you started the mill.

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Waiting on 2 100Mfd run capacitors to complete my RPC. Tried using the Fitch plans but couldn't follow the schematic so I noodled out my own. Would appreciate any input you folks may have before I apply power to the unit. Would hate to let the magic smoke out. LOL

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H-M Supporter - Gold Member

It looks like it will work, myself I probably would have used one double pole start button rather than two separate start buttons
mark
Question: are you rebuilding your existing unit from scratch using the original motor?

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For markba663csi, I am rebuilding my original unit using the original motor. The motor shop checked it and proclaimed it as good. When I picked it up they ran it for me however they had it wiried differently than I had it wired. They had it wired star/delta, meaning it would start as wye/star then run as delta. It is a 12 wire motor and can be run on either 230 or 460 power. The reason I didn't use a double pole switch is that I was afraid the start capacitor would stay energized after the motor was started.. I hooked it up today but couldn't get it to run. It kept tripping the main breaker. Am going to have a friend, who is an electrician, look it over and see if he can figure out where I screwed up.

For JimDawson, I didn't think there would be that much load on the start capacitor as it was only going to be energized for a few seconds. How much current do you think it is drawing for those few seconds?

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For JimDawson, I didn't think there would be that much load on the start capacitor as it was only going to be energized for a few seconds. How much current do you think it is drawing for those few seconds?